feat: robust automatic DB migration for Docker deployments

Backup before migration (pg_dump/SQLite copy), DB connection wait with
retries, idempotent prisma migrate deploy, old backup cleanup (keep 5),
and server refuses to start if migration fails.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

docker-compose.yml

@@ -41,6 +41,7 @@ services:
       - NEXT_TELEMETRY_DISABLED=1
     volumes:
       - uploads-data:/app/public/uploads
+      - backup-data:/app/data/backups
     depends_on:
       postgres:
         condition: service_healthy
@@ -136,6 +137,8 @@ volumes:
     driver: local
   uploads-data:
     driver: local
+  backup-data:
+    driver: local
   ollama-data:
     driver: local